The present invention relates to a method of detecting focus conditions for use in a still camera, a cinecamera, a single-lens reflex camera, a photomicrography taking device or the like and a device for carrying out the method.
Various systems for detecting focus conditions have been proposed. For example, a defocused detecting system exists which utilizes an evaluation function for calculating defocused amount (sharpness) of the image. FIG. 1 shows a construction of a focus condition detecting device for the single-lens reflex camera adopting such a defocused image detecting system. The light beam from an object lens 1 is divided in part or in whole into two parts by a quick return mirror 2. One of the divided beams is led to a finder system (not shown) and the other of the dividing beams is led to a beam splitter 4 by a total reflection mirror 3 which is arranged on the rear surface of the mirror 2. This latter divided beam is further divided into two parts by the beam splitter 4. These latter divided beams are projected onto two accumulative type light receiving element arrays 5 and 6 such as photocell arrays, for example, CCD, PDA or the like. So as to form a predetermined optical path difference in the optical axis direction, photocell arrays 5 and 6 are equidistantly arranged in the front and the rear of a plane conjugated to a film surface thereby forming an image corresponding to the position in the optical axis direction of the lens 1 onto these photocell arrays. Outputs photoelectrically converted by the photocell arrays 5 and 6 are analog/digital (A/D) converted by an A/D conversion circuit 7 and arithmetically operated upon by a central processing unit (CPU) 8 based on a predetermined evaluation function which calculates sharpness of image. This results in respective evaluation values corresponding to a decision of focus conditions such as front focus, in-focus and rear focus. As the evaluation function, if the output of the ith element of a photocell array is X.sub.i, a sum of the maximum value .vertline.X.sub.i -X.sub.(i-1) .vertline..sub.max of, for example, .vertline.X.sub.i -X.sub.(i-1) .vertline. and the next largest value .vertline.X.sub.i -X.sub.(i-1) .vertline..sub.submax is utilized.
FIG. 2 shows a relation between the position of lens 1 and respective evaluation values obtained by arithmetically operating upon the outputs of photocell arrays 5 and 6 utilizing the evaluation function S=.vertline.X.sub.i -X.sub.(i-1) .vertline..sub.max +.vertline.X.sub.i -X.sub.(i-1) .vertline..sub.submax. Solid line S.sub.1 shows an evaluation value obtained from photocell array 5 and broken line S.sub.2 shows an evaluation value obtained from the photocell array 6. Respective evaluation values S.sub.1 and S.sub.2 of photocell arrays 5 and 6 have maximum value when in-focused condition is obtained on each photocell array and evaluation values of both photocell arrays are equal to each other when in-focused condition is obtained on the film surface. Respective evaluation values S.sub.1 and S.sub.2, therefore, are obtained at any position of the lens 1 and compared with each other thereby deciding the rear focus condition in the case of S.sub.1 &gt;S.sub.2, the in-focused condition in the case of S.sub.1 =S.sub.2, and the front focus condition in the case of S.sub. 1 &lt;S.sub.2, so that manual or automatic focus adjusting can be performed according to the decided result.
The above conventional focus condition detecting device can perform focus condition detecting with high precision by a comparatively simple optical system. But as seen from FIG. 2 the range in which evaluation values S.sub.1 and S.sub.2 are changed is remarkably narrow, and thus in the region that the lens 1 is remarkably separated from the in-focused position, evaluation values S.sub.1 and S.sub.2 are scarcely changed and become substantially equal to each other so that the region within which a focus condition can be properly detected becomes narrow and the detection of defocused direction is impossible.